U. S. Pat. No. 3,453,389 issued to N. R. Shaer on July 1, 1969 discloses an improved telephone switching system having facilities for serving customer-dialed, station paid calls originating at a coin telephone station. In particular, Shaer discloses equipment at a traffic service position system (TSPS) telephone office comprising: (1) recording announcement apparatus, which automatically transmits time and charge information to a calling coin station, and (2) a coin deposit monitor, which detects and utilizes tones generated at the calling station, in response to each coin deposit, to determine whether the amount deposited matches the amount requested. The announcement apparatus and the coin deposit monitor are utilized during various coin collection intervals, including the beginning of a call for the collection of an initial time duration deposit, the beginning of each overtime duration for the collection of interim overtime charges, and the termination of a call for the collection of postpay overtime charges.
Typically, the speech network in a coin telephone station remains connected to the telephone office during the coin collection intervals. Accordingly, a fraudulent coin deposit signal, acoustically coupled through the speech network at the coin telephone station, may compromise the collection process by falsely indicating the coin deposit amount. To help overcome such acoustically coupled signals, the speech network transmitter can be isolated from the telephone line during the duration of the coin collection interval in which stored coin deposit information is outpulsed to the telephone office. The isolation is accomplished in known arrangements by inhibiting the speech network transmitter, for example, by disconnecting the transmitter from the line, or by attenuating any acoustically coupled signals via a high impedance in the speech network transmitter. The out-pulsing, typically either multifrequency or dial pulse, is delayed until a special interrogation signal, received from the telephone office, is detected by an interrogation signal detector at the coin station.
An inherent problem with such known arrangements is that the signal detector must be capable of detecting an interrogation signal substantially below a local average signal power level. Further, the signal detector typically requires circuitry having high selectivity and high stability. Consequently, known arrangements using interrogation signal detectors are usually prohibitively expensive for use in the coin telephones of a large telephone system.
It is therefore a general object of the invention to provide an improved coin telephone station for serving coin calls.
It is a more specific object of the invention to obviate an acoustically coupled coin deposit signal, fraudulently inserted at a coin telephone station.
A further object of the invention is to obviate acoustically coupled coin deposit signals without using an interrogation signal and detector arrangement.